A hydrodynamic bearing device, which utilizes the fluid pressure of an oil or other lubricating fluid interposed between a shaft and a sleeve to support the two so as to be capable of relative rotation, has been proposed as a bearing for spindle motors, which are used in recording apparatus for rotationally driving a disk-shaped recording medium, such as a magnetic disk, optical disk, or magneto optical disk.
A series of microscopic clearances are formed between the shaft and the sleeve, and a hydrodynamic groove that is formed in the peripheral direction of the rotary shaft (called a radial hydrodynamic groove), and a hydrodynamic groove that is formed in the radial direction of the rotary shaft (called a thrust hydrodynamic groove), are formed in the shaft and/or the sleeve. Oil is held as a lubricating fluid in these microscopic clearances. One type of such hydrodynamic bearing devices has a structure in which a taper seal is formed at the end of the series of microscopic clearances and exposed to the atmosphere, which is called a single-pocket structure.
With the single-pocket hydrodynamic bearing device of Patent Document 1, the shaft has a large-diameter flange at the lower end, and a flange cover is disposed opposing to the flange. A thrust hydrodynamic groove is formed in the flange and/or the flange cover. The faces are flat everywhere except in the portion where the thrust hydrodynamic groove is formed.
With a conventional hydrodynamic bearing device structured like this, when the shaft begins to rotate, the lubricating fluid is drawn into the radial and thrust bearing portion by the pumping pressure generated by the hydrodynamic grooves, and the fluid pressure rises within the hydrodynamic grooves. This results in a state in which the shaft and sleeve are able to rotate relative to each other without contact.
In some cases, communicating holes are provided in the inner periphery of the flange, as seen in Patent Document 2, for example, in order to eliminate the pressure imbalance at the top and bottom of the thrust bearing portion during rotation.
Patent Document 1: Japanese Laid-Open Patent Application 2005-308057
Patent Document 2: Japanese Laid-Open Patent Application 2003-28147